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Islamic State fanatics and other terror groups are planning another massive attack on the scale of 9/11, a top US security chief warned today.

Elaine Duke, Donald Trump's acting Secretary of Homeland Security, said jihadists were using crude knife and van attacks to keep their members engaged and their finances flowing as they plot another "big explosion" similar to the September 2001 atrocities.

Speaking at the US embassy in London, she said intelligence is pointing to extremists plotting to take down planes to inflict mass civilian casualties, according to the Daily Mail.

The security chief, who has served three US presidents, said: "The terrorist organisations, be it ISIS or others, want to have the big explosion like they did on 9/11. They want to take down aircraft, the intelligence is clear on that.

"However, in the interim they need to keep their finances flowing and they need to keep their visibility high and they need to keep their members engaged, so they are using small plots and they are happy to have small plots."

She added: "Creating terror is their goal and so a van attack, a bladed weapon attack, causes terror and continues to disrupt the world - but does not mean they've given up on a major aviation plot."

Yesterday Duke said the prospect of a terrorist blowing up an airline using a laptop was just one of the threats facing airlines worldwide.

She said the free movement of goods and people means security has to be tightened in individual countries around the world.

She said: "The laptop is one of the many aviation threats, we will never be comfortable and we will always be evolving.

"What we believe is that because of the movement of goods and people, we have to raise the baseline worldwide, we can't only consider our borders." Duke went on: "We think the level of terrorist threat against the United States too is extremely high.

"I think that it is challenging for you because you have the proximities to other countries, the ease of movement from some of the terrorist safe havens is a little easier for you, but we feel the terrorist threat is very high in the United States."

Islamic State fanatics and other terror groups are planning another massive attack on the scale of 9/11, a top US security chief warned. Photo / AP

Asked how the US is tackling the threat of another 9/11-style atrocity, she said: "We have worked on some strong measures that we can't talk about. We are trying to play the away game and that is working against them in their terrorist safe havens and homes.

"We do have some terrorist groups on the move, you just saw the take-over of Raqqa and so if we can keep them declining and moving they have less time to sit and prepare."

Duke warned that the number of home-grown violent extremists, mostly inspired by terrorist organisations, is increasing in the US. She said the ability of IS militants to put terrorist propaganda on the internet will appeal more and more to extremists as they are pushed out of Syria and Iraq.

Duke said web giants need to do more to detect extremist content online, and one way of doing this could be using the same technology used to identify people in passenger lists.

"Terrorists are strong, they are adaptable and the terrorist threat is the highest it has been since pre-9/11. We have got to have every tool that's possible," she added.

Hijacked United Airlines Flight 175 from Boston crashes into the south tower of the World Trade Center and explodes. Photo / Getty Images

A total of 2,996 people were killed during the September 11 attacks, when al-Qaeda suicide attackers hijacked planes and flew them into the World Trade Center in New York and the Pentagon in Washington.

Earlier in the day she met the British interior minister Amber Rudd to discuss how to force internet giants to do more to tackle terrorism ahead of the G7 summit.

Following the recent wave of attacks in Manchester and London, police chiefs have said the threat facing the UK is a "new norm" that will not change.

Her chilling remarks came 24 hours after MI5 director general Andrew Parker warned Britain is facing its worst-ever terrorist threat in his first major speech since the UK was hit by a wave of attacks.

The British spy chief said it was taking terrorists just days to hatch plots as violent extremists exploit "safe spaces online" to evade detection.

It is harder for the UK to protect itself because of its proximity to other countries and the ease of movement from terrorist safe havens, she suggested.